All the World's a Stage
... for Dust

Tune in to a NASA website and watch giant dust clouds
as they ride global rivers of air, cross-pollinating continents
with topsoil and microbes.

June 26, 2001 -- People may think of the ground under
their feet as being stationary, but the soil and dust of the
world is constantly on the move, blown aloft by the wind -- sometimes
across entire oceans!

Now, anyone with access to the World Wide Web can watch airborne
dust migrate around the globe simply by pointing their browser
to the aerosol
homepage for NASA's Total Ozone Mapping
Spectrometer, or "TOMS" for short. Although
the primary function of TOMS is to monitor the ozone layer, it
also measures aerosols
-- that is, airborne dust, smoke, and other particulates. NASA
scientists use these data to create daily maps and movies that
they post online.

Above: This TOMS aerosol movie, which
spans the interval June 13 through 21, 2001, shows a cloud of
African dust crossing the Atlantic and raining bits of the Sahara
Desert over the Caribbean. Click on the image to view a larger
450 kb animation,
which includes a color table and legend. See also the
latest TOMS maps and more information about the color-coded
aerosol
index.

Our planet's atmosphere provides a transcontinental highway
for dust that's been stirred up from dry soils by strong winds.
Because dust particles are so small -- often less than 0.002
mm across -- they can remain aloft for days as they ride global
rivers of air. Larger sand grains don't get airborne as often
or for as long, but they can be pushed along the ground by the
wind or washed away by water erosion.

This constant reshuffling of the world's sand and dust
ties the continents together and serves as a reminder that, in
the natural world, there are no political boundaries. Airborne
microbes and pollen in Florida or Brazil might have come
from Africa. Mineral dust in the soils of India could have blown
over from Iran.

In fact, over vast stretches of geologic time, the action
of wind thoroughly mixes and re-mixes the world's dust. So the
soil in your own backyard might contain some grains of dust from
places all over the globe!

While much of this reshuffling of earth can't be seen by satellites,
dust often migrates in huge clouds that show up in satellite
images.

For example, the Americas frequently inherit huge volumes
of dust from Africa and Asia, while countries of the Far East
like India and China get sprinkled with dust from the Middle
East.

African dust plumes begin their trans-Atlantic journey with
storm activity in the Sahara Desert region. The dust, originating
from fine particles in the arid topsoil, is transported into
the atmosphere by winds and may be carried more than 10,000 feet
high. Dust clouds cross the Atlantic Ocean and reach the Caribbean
and the Americas in about 5 to 7 days.

How much dust is transferred by these clouds?
One estimate places the number at about 13 million tons of dust
each year drifting from Africa to the Northeastern Amazon Basin
alone!

Right: This TOMS image shows a record-setting
Asian dust cloud beginning its journey east across the Pacific.
Click on the image to view a 440 kb animation
of the dust cloud migrating to North America. Remember that as
the dust disappears from the satellite's view, it's raining out
of the air onto the ocean and the land. See a similar Asian dust
cloud from 1998 in this animation.

"In some cases, such as in the Caribbean islands, much
of the dust in the top soil is from Africa," says Jay Herman,
principle scientist for aerosol detection by TOMS at NASA's Goddard
Space Flight Center. "Over generations, lots of it piles
up there. The top soil has been analyzed on these islands, and
it's definitely of African origin."

Northern spring also brings massive dust clouds blowing east
over the Pacific Ocean from China and Mongolia. The largest
Asian dust cloud on record blew all the way across North
America earlier this year, traveling as far east as the mid-Atlantic
Ocean before finally disappearing from view.

"The huge amount of dust blowing off Asia recently is
kind of a new phenomenon," Herman says. "It's apparently
due to increased desertification brought about by a change in
the dryness of the area and increased land use."

Usually, dust clouds from Asia don't reach so far, and much
of the dust simply falls into the Pacific Ocean. Asian dust clouds
don't make it to the Americas as often as African clouds do for
two main reasons, Herman says. First, the season for dust-carrying
winds is much shorter in China than it is in Africa. Second,
the sources of Asian dust -- such as the Gobi and Takla Makan
deserts -- are much smaller than the African sources, which include
the Sahara Desert and the ever-shrinking
Lake Chad.

Below: The hazy streets of Baichen
in the Jilin Province of northern China appear somewhat apocalyptic
on April 7 during the peak of the violent dust storms that gave
birth to the record-setting Asian dust cloud. Click on the image
for a larger
version. Photo by Zev Levin.

The raining of dust into
the Pacific Ocean may be important for the productivity of fisheries
in the North Pacific, an important source of seafood for much
of the world. Photosynthetic
microbes in the top layer of the ocean form the foundation
of the food chain. Often the concentration of dissolved mineral
nutrients (particularly iron) controls the abundance of these
microbes. Some of the minerals in the dust might help boost microbe
populations and, therefore, fish populations. Scientists are
still working to verify this effect, however.

Airborne dust clouds can also affect local weather by suppressing
rainfall. Cloud droplets form around the minute dust grains,
but some kinds of dust limit
how big these droplets can become. Because droplets must reach
a certain size before they'll fall to Earth as rain, dust clouds
can leave dry weather in their path.

In addition to dust, the TOMS instrument detects volcanic
ash, pollutants, smoke, ozone concentrations and ultraviolet
light intensities. Most of these data are available online in
near real-time.

Remarkably, the sensor is able to make all of these measurements
by simply observing the sunlight reflected toward it from the
Earth. "It looks at the Earth just as you would with
your eye to see how bright it is in the ultraviolet
wavelengths," explains Herman. "If you could
see in the ultraviolet, you'd basically be seeing a color change
when aerosols are present. That's all the measurement is."

Right: Dry soil on the move. Dust
storms can be a frightening sight -- they kick up walls of dust
often hundreds of feet high that sand-blast everything in their
path. Some of this dust could get lofted up to higher altitudes,
where streams of air could carry it hundreds of miles and deposit
it on distant soils.

To detect aerosols, the TOMS instrument records two particular
wavelengths of ultraviolet (UV) light: 331 nanometers and 360
nanometers. Some of the UV light reflecting from Earth is absorbed
by the dust and other aerosols. Clouds reflect both wavelengths
almost equally, while aerosols cause absorption of the shorter
wavelength more than the longer one. This difference allows scientists
to distinguish aerosols from clouds.

"We put data on our website every day, usually within
minutes of receiving it," Herman says.

A new TOMS instrument is scheduled to launch on August 10
aboard the QuikTOMS
satellite. This satellite will replace the aging TOMS instrument
currently in orbit, and while both instruments are functional,
the redundant data will help NASA scientists to calibrate the
new one. QuikTOMS's expected mission life is 3 years.

Thanks to TOMS and its successor, we'll continue to have a
front row seat on the global stage of migrating dust for years
to come.

April 2001 Asia dust event -- information about the largest dust
cloud to cross the Pacific Ocean from Asia on record, from the
TOMS website

Mobile Homes for Microbes -- Science@NASA story
African dust that crosses the Atlantic and brings beautiful sunsets
to Florida also carries potentially harmful bacteria and fungi,
a new study shows.

The Pacific Dust Express -- Science@NASA story
North America has been sprinkled with a dash of Asia! A dust
cloud from China crosed the Pacific Ocean recently and rained
Asian dust from Alaska to Florida.

Dust Begets Dust -- Science@NASA
story Everyone knows
that dry weather leads to dusty soils, but new research suggests
that dust might in turn lead to dry weather.

Watching Wildfires from Space -- Science@NASA story
Find out more about Earth Probe TOMS and how it keeps track of
dust and smoke in Earth's atmosphere.